a. Field of Invention
The invention concerns an ultrasonic sensor with a polymer foil fastened to a support structure at least at its peripheral area and which is piezoelectrically activated at least in part which is electrically coupled to electrodes.
b. Description of the Prior Art
Devices known as miniature or membrane hydrophones are used for the determination of the properties of an ultrasonic field existing in a sound-carrying medium, for example water. The three-dimensional distribution of the acoustic pressure amplitude of the ultrasonic field is determined by measuring the acoustic pressure existing in a measuring container at various sites with such a hydrophone.
A miniature hydrophone is known from "Ultrasonics", September 1981, pp. 213 to 216, which comprises piezoelectric polyvinylidene fluoride PVDF foil with a thickness of 25 um (micrometers) and equipped with electrodes on its two flat sides and which is stretched across and electrically insulated from the front end of a refined steel tube. The diameter of the foil is approx. 1 mm. A platinum wire connected to the inner conductor of a coaxial cable is attached on the inside of the foil. This platinum wire is supported by a non-conductive material filling the inside of the refined steel tube. The outside of the foil is in electrical contact with the refined steel tube and connected to the shielding of the coaxial cable.
A membrane hydrophone with a polyvinylidene fluoride PVDF foil with a thickness of 25 um stretched between two metal rings serving as support structures is disclosed in "Ultrasonics", May 1980, pp. 123 to 126. A membrane with an inside diameter of approx. 100 mm is formed thereby. The surfaces of the membrane are equipped with circular disk-shaped electrodes facing each other in a small, central area, and the diameter of the electrodes is 4 mm, for example. The polarized, piezoelectrically active area of the membrane is located between these electrodes. Connecting leads attached in the form of metal films to the surfaces of the membrane lead from the circular disk-shaped electrodes to the edge of the membrane, where they make contact with a coaxial cable through a conductive adhesive.
A significant advantage of these types of hydrophones is that the acoustic impedance of their piezoelectric elements matches better the acoustic impedance of water than with the use of ceramic piezoelectric materials. In comparison to ceramic sensors, an increased width of the frequency band as well as a decrease in the interference with the ultrasonic field at the measuring site results.
But ultrasonic shock waves with high pressure amplitudes in the range approximately 10.sup.8 Pa cannot be measured with such hydrophones. This type of shock waves with very steep pulse fronts that have rise times below 1 us (microsecond) lead to a mechanical destruction of the metal electrodes attached in the piezoelectrically active area of the PVDF foil of the known hydrophones due to cavitation effects. Such shock waves occur, for example, in the focal area of lithotriptors using a focussed ultrasonic shock wave for the shattering of concretions, for example kidney stones in the kidney of a patient. The properties of the shock wave in the focal area must be determined for the development as well as for the routine monitoring of such devices.